My laboratory is taking molecular, genetic and biochemical approaches to understand the molecular mechanisms that control cell size. Using the fruitfly Drosophila melanogaster as an experimental model, my laboratory has identified two prominent signaling pathways that act in concert to regulate cell size. One of them is the insulin receptor/PI3K pathway. The other pathway is the Tsc1/Tsc2/TOR signaling pathway. Activation of either pathway leads to increased cell size. Unlike the insulin pathway, which is normally used to couple cell growth to intercellular hormones such as insulin, the Tsc1/Tsc2/TOR pathway is mainly regulated by nutrient availability. Recent studies from my laboratory have identified Rheb, a small GTPase of the Ras superfamily, as a novel component of the Tsc1/Tsc2/TOR nutrient-sensing pathway, thus providing a new entry point to understand the molecular architecture of this pathway. The focus of the current proposal is to understand the function and regulation of Rheb with respect to the Tsc1/Tsc2/TOR pathway. Three specific aims are proposed. First, we will determine the molecular mechanisms by which TOR is regulated by Rheb. Second, we will identify the guanine nucleotide exchange factor(s) for Rheb. Third, a genetic screen will be conducted to identify additional components of the Rheb signaling pathway in cell growth. These studies should not only help us understand basic developmental mechanisms, but also provide insights into human diseases and cancer.